Ink pen having a hydrophobic barrier for controlling ink leakage

Abstract: An ink pen 10 is provided with a hydrophobic membrane 19 to control the leakage of ink 14. The ink pen has a vent 18, such as a bubble generator, to allow the ingress of air into the ink reservoir 12 and thereby regulate the backpressure within the reservoir. The hydrophobic membrane which allows the flow of air but prevents the flow of ink is positioned within the vent to control leakage of ink from the ink pen through the vent.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to ink pens for ink-jet printers, and more particularly, to an apparatus for controlling ink leakage from the reservoir of an ink pen. 2. Description of Related Art Ink-jet printers have become established as reliable and efficient printing devices. Typically, an ink-jet printer utilizes a print head which is moved relative to a printing surface. A control system activates the moving print head at the appropriate locations causing the print head to eject, or jet, ink drops onto the printing surface to form desired images and characters. Such printers typically include an ink pen which serves as a reservoir for storing ink and provides a means of supplying ink, as needed, to the print head. There are two commonly used systems for ejecting ink from a print head. The first is a thermal bubble system and the second is a piezoelectric system. A print head using either system typically includes a plurality of orifices, each orifice having an associated chamber. In operation, ink is supplied via an inlet to the chamber. Upon activation, the ink is forced, or jetted, from the chamber through the orifice and onto the printing surface. In thermal bubble type print heads, the ink in the chamber is heated or vaporized, typically by a thin film resistor. The rapid expansion which results from vaporization of the ink forces a quantity of ink from the chamber through the orifice. In piezoelectric type print heads, a piezoelectric element creates a pressure wave within the chamber which ejects a quantity of ink through the orifice. Although both thermal bubble and piezoelectric print heads provide a reliable and efficient means of jetting ink from an orifice, both types of print heads generally have no mechanism to prevent the free flow of ink through the orifice when the print head is not activated. If this occurs, ink may leak, or drool, uncontrollably through the print head. Typically, printers are provided with catch basins to catch and contain ink dripping from the print head. This helps to prevent the ink from damaging the printer. However, the ink may drip onto the printing surface to produce an undesirable ink spot. In addition, leaking ink may build up on the print head and impair the proper operation of the print head. In any case, a leaking ink pen will usually need to be discarded and replaced. To alleviate these problems, many ink-jet printers supply ink from the ink pen to the print head at a slight underpressure or backpressure. As used herein a positive backpressure is used to refer to a pressure within an ink pen that is lower than the ambient pressure surrounding the print head orifice. To be effective, the backpressure must be maintained within a desired operating range. That is, the backpressure must be large enough to prevent the unwanted free flow of ink through the orifice. At the same time, the backpressure must be small enough that the print head, when activated, can overcome the backpressure and eject the ink in a consistent and predictable manner. To meet these constraints and provide optimum operation of the ink-jet printer, a fairly constant and predictable backpressure should be maintained. The backpressure of an ink pen is affected by changes in either the ambient pressure or the internal pressure. For example, if an ink pen is subject to an increase in altitude, such as during transport aboard an aircraft, the ambient pressure may decrease substantially. Unless the backpressure of the ink pen increases accordingly, the ambient pressure level may drop below that of the backpressure and ink will likely leak from the print head. In addition, as ink is depleted from the ink pen reservoir the backpressure within the ink pen will tend to increase. Without some mechanism to compensate for this, the backpressure may exceed the operating range of the print head and the ink pen will become inoperative. Temperature variations may cause the ink and air within the ink pen to contract or expand, thereby affecting the backpressure. All of these factors must be accounted for in order to ensure consistent trouble-free operation of the ink-jet printer. One type of ink pen uses an expandable bladder in conjunction with a vent to maintain the proper backpressure within an ink-jet pen. The expandable bladder is situated within the reservoir and configured to expand or contract in response to depletion of ink from the reservoir, pressure changes, temperature variations, or the like. Typically, the bladder is biased with a spring or some similar mechanism which resists expansion of the bladder. This resistance helps to maintain a back pressure within the reservoir. In conjunction with the expandable bladder, some pens incorporate a vent. The vent is typically configured to selectively allow the entry of atmospheric air into the ink reservoir when the backpressure reaches an undesirable level. The ingress of air through the vent lowers the backpressure. In this manner, the biased expandable bladder services to create the necessary backpressure and the controlled ingress of air through the vent prevents the backpressure from exceeding the desired range. In EP-A-0508125 there is disclosed an ink delivery system for an ink jet printer. The system includes a liquid compartment and a hydrophobic membrane mounted between the liquid compartment and the surrounding environment. The membrane allows ambient air to bubble into the liquid while preventing liquid from flowing in the opposite direction through the membrane. The liquid compartment is in gaseous communication with at least one ink chamber and, as a result, a backpressure in the ink chamber is regulated by the flow of air through the membrane. The combination of an expandable bladder and a vent has proven to be an efficient and effective mechanism for creating and maintaining the desired backpressure within the reservoir of an ink pen. However, under extreme environmental conditions, or in the case of failure of the expandable bladder or a breach of the integrity of the ink reservoir it is sometimes possible for the backpressure in the ink reservoir to drop below the desired range. In some cases, such conditions may even create a negative backpressure (that is, a pressure within the reservoir that is higher than ambient) within the ink reservoir. Should this occur, it is possible for ink to be forced from the reservoir. Ink forced from the reservoir will typically exit through either the print head or the vent. As discussed above, printers are typically equipped to minimize damage from ink leaking through the print head. On the other hand, ink leaking through the vent can have disastrous consequences. In some printer configurations, no catch basin is provided to catch ink leaking from the vent. Moreover, given the usual location of the vent, ink dripping from the vent can land directly on exposed electrical circuits and electrical contacts. If this occurs, the printer may be severely damaged. In EP-A-0529880 there is disclosed an orientation sensitive valve for an ink-jet pen. The valve is operable with a bubble generator that provides a restricted flow of air bubbles into a reservoir of an ink-jet pen to relieve excessive backpressure in the pen. The valve includes a chamber provided with operating liquid that closes the valve in the event that the pen is tipped out of an upright orientation. The liquid is in communication with ambient air and, together with an inlet passage forms the bubble generator. A hydrophobic air-porous vent located between the chamber and the reservoir is provided for permitting passage of air bubbles from the chamber and into the reservoir while prohibiting the flow of the operating liquid. US-A-5010354 describes an ink-jet pen in which a reservoir contains free ink and the backpressure in the reservoir is maintained over environmental excursions by a capillary element in a chamber. A bubble generator allows air to be introduced into the reservoir. In an air passageway leading from the ambient air is an ink filter foam to ensure that ink does not leak out. Accordingly, it is an object of the present invention to provide an ink pen having a mechanism for controlling ink leakage from an ink pen without impairing the function and operation of the ink pen. It is a further object of the invention to provide an apparatus for controlling ink leakage from an ink pen that is easy and inexpensive to manufacture and has few complicated parts. In accordance with the present invention there is provided an ink pen for an ink jet printer as specified in claim 1 hereinafter. A reservoir is provided with a vent, such as a "bubble generator," for allowing the ingress of air into the reservoir. A hydrophobic membrane that blocks the flow of ink and allows the flow of air is positioned in the vent to prevent ink from flowing out of the reservoir through the vent. Other objects and aspects of the invention will become apparent to those skilled in the art from the detailed description of the invention which is presented by way of example and not as a limitation of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a partially exploded, bottom, perspective view of an ink pen in accordance with one embodiment of the present invention. Figure 2 is bottom view of the ink pen of Figure 1. Figure 3 is a cross sectional view taken along line 3-3 in Figure 2. Figure 4 is an enlarged view of a portion of Figure 3 showing the hydrophobic vent. DESCRIPTION OF THE ILLUSTRATED EMBODIMENT An ink pen in accordance with a preferred embodiment of the present invention is illustrated in Figure 1 as reference numeral 10. The ink pen 10 has a reservoir 12 for storing a supply of ink 14. The reservoir is in fluid communication with a print head 16 which ejects ink drops onto a printing surface to form characters and images. The ink within the reservoir is subject to an initial backpressure to prevent the ink from drooling through the print head. The initial backpressure is created and maintained with the aid of a biased expandable bladder (not shown) positioned within the ink reservoir. Any one of a number of known expandable bladder structures may be used, so long as the expandable bladder can respond to environmental changes, depletion of ink from the reservoir, or the like, to help regulate the backpressure within the reservoir. The reservoir 12 is provided with a bubble generator 18 which allows air to enter the reservoir in a controlled manner to regulate the backpressure within the reservoir. A hydrophobic membrane 19 is positioned in the path of the bubble generator. The hydrophobic membrane 19 allows the passage of air and blocks the passage of ink. In this manner, the hydrophobic membrane prevents ink leakage from the ink pen through the bubble generator while allowing the free flow of air necessary for the proper operation of the bubble generator. As shown best in Figure 3, the illustrated bubble generator 18 consists of a tubular boss 22 formed in the bottom wall of the reservoir. One end 21 of the boss 22 extends into the reservoir where it is open to allow ink to enter the boss. The other end 23 of the boss 22 opens to an inlet labyrinth 30 through which air can enter the boss. A sphere 24 is mounted concentrically within the boss 22 to divide the first end 21 from the second end 23. The outside diameter of the sphere 24 is smaller than the inside diameter of the boss 22 such that the sphere and boss define an annular orifice 20. A plurality of raised ribs 25 on the inside of the cylindrical boss 22 engage the sphere 24 to maintain it in position within the boss. Normally, a quantity of ink is trapped within the annular orifice 20 to prevent the ingress of air through the bubble generator. The ink trapped within the orifice 20 is supplied from the reservoir. In its normal orientation the boss 22 is submerged in the ink until the reservoir is nearly depleted. This allows a quantity of ink from the reservoir to enter the boss to seal the orifice. In other orientations, or when the ink reservoir is nearly depleted, the sphere 24 serves as a capillary member to maintain a quantity of ink within the boss 22. As a result, even when the pen is oriented such that the boss is not submerged in the reservoir ink, a quantity of ink is trapped within the boss 22 to seal the orifice 20. Due to the curved surface of the sphere 24, the gap between the exterior surface of the sphere and the inner wall of the boss is smallest at the orifice 20 and increases as the distance from the orifice increases. This geometry, coupled with the capillarity of the ink, constantly urges the trapped quantity of ink toward the orifice--the smallest portion of the gap--to provide a robust seal. However, if the backpressure within the pen exceeds a particular level, the capillary forces holding the ink within the annular gap are overcome by the pressure gradient across the bubble generator and air is allowed to bubble through the trapped ink to thereby lower the backpressure. The particular backpressure level at which any given bubble generator will admit air is a function of the material which the boss and sphere are made of, the size and geometry of the annular orifice, the viscosity and surface tension of the ink, and other similar factors. These factors are typically selected such that the bubble generator prevents the backpressure within the reservoir from exceeding the operating range of the ink pen. To prevent the trapped quantity of ink from drying or solidifying as a result of prolonged exposure to the atmosphere, the bubble generator is provided with an inlet labyrinth 30 which serves as a vapor barrier. The inlet labyrinth 30, best seen in Figures 1 and 2, is a path through which the ambient air must travel before contacting the trapped ink. The proximal end 31 of the labyrinth opens to the boss and the distal end 33 is covered with the hydrophobic membrane 19 and open to the ambient air through hole 36. The length of the labyrinth is sealed from both the ambient and the reservoir. As a result, the humidity within the labyrinth varies along its length from approximately 100% at the proximal end 31 to approximately ambient at the distal end 33. This humidity gradient serves to shield the trapped ink from direct contact with ambient air and prevent the trapped ink from drying or solidifying. The inlet labyrinth 30 also serves as an overflow receptacle. If the ink pen is subject to an extreme environmental change, or if the expandable bladder fails causing the backpressure within the reservoir to drop below the level necessary to prevent ink from leaking through the annular orifice 20, the ink can exit the reservoir via the bubble generator and enter the inlet labyrinth 30. The hydrophobic membrane 19 prevents the ink from leaking from inlet labyrinth through hole 36. Subsequently, when conditions return to normal, the ink in the inlet labyrinth can reenter the reservoir. The hydrophobic membrane 19 is made of a material which allows air to pass but which blocks the flow of ink. In this manner, the hydrophobic membrane 19 prevents any ink which enters the inlet labyrinth 30 through the bubble generator 18 from leaking from the ink pen. At the same time, the hydrophobic membrane 19 allows the flow of air through the hole 36 to the bubble generator 18 to ensure its proper operation. In the illustrated embodiment, a material sold under the designation Pall Flex™ JO1426W has been found to be a satisfactory hydrophobic membrane. However, other materials may also work. An appropriate material should allow an adequate flow of air to ensure proper operation of the bubble generator. At the same time, the hydrophobic material must block the flow of ink to prevent ink from leaking from the pen through the bubble generator. In the illustrated embodiment, the material preferably allows the flow of air through the hole 36 at a rate of about 5.5 cubic centimeters per minute per square millimeter with a pressure drop of less than about 1.3 centimeters water column. The material in the illustrated embodiment also preferably blocks the flow of ink up to a pressure of at least about 51 centimeters water column. In addition, the material preferably allows ink to be easily removed from its surface. This characteristic helps to allow ink within the labyrinth to return via the bubble generator to the reservoir when the proper backpressure is restored. In the illustrated embodiment, it is preferable that ink can be removed from the membrane with a pressure of less that about 20.4 centimeters water column. It is also preferable that the material resist the absorption and saturation of ink. Otherwise, when the backpressure is restored, the material may not allow the free flow of air necessary for the bubble generator to function properly. As seen in Figures 1, 2, and 3, the inlet labyrinth in the illustrated embodiment, is a trough 32 molded directly into the external surface of the reservoir 12. The exact dimensions of the trough are chosen to ensure an adequate humidity gradient to prevent the liquid seal of the bubble generator from drying out. In the illustrated embodiment, the trough is about .64 millimeters deep and about .64 millimeters across. A cover 34 is attached to the external surface of the reservoir over the trough 32 to seal the length of the trough. A hole 36 corresponding with the distal end of the trough 32 is provided in the cover 34 to allow air to enter the trough. The hydrophobic membrane 19 is attached to the inside of the cover 34 over the hole 36. To receive the hydrophobic membrane, the distal end of the trough is provided with a well 42. In order to ensure a good seal around the well when the cover is attached, it is preferable that the well be larger than the diameter of the hydrophobic material so that the hydrophobic material does not contact the edges of the trough. Three support columns 44 are formed in the well 42 to support the span of the cover 34 and the hydrophobic membrane over the well. In the illustrated embodiment, the hydrophobic membrane is attached to the underside of the cover by heat staking. That is, the hydrophobic membrane is placed in position adjacent the cover and a heated element is brought into contact with the hydrophobic material. This causes the cover, which is preferably made of polysulfone, to melt and fuse to the hydrophobic membrane. Preferably, the bond between the hydrophobic material and the cover is formed at the periphery of the hydrophobic membrane. This maximizes the area of the hydrophobic membrane through which air is allowed to pass. In a preferred method of attaching the hydrophobic membrane to the cover, the heated element is provided with a raised burr corresponding to the desired outline of the hydrophobic membrane. A strip of hydrophobic material is placed over a cover and the heated element is brought into contact. As pressure is applied, the burr of the heated element simultaneously cuts the hydrophobic material to form the hydrophobic membrane and heat stakes the periphery of the hydrophobic membrane to the cover. In the illustrated embodiment, the cover is attached to the reservoir body by ultrasonic welding. A raised ridge 40 surrounding the trough (seen only in Figure 2) serves as an energy director to facilitate the welding process and seal the trough. The cover is positioned over the trough by means of alignment pins 46. Once in place, the ultrasonic welding horn is brought in contact with the cover. The welding apparatus then causes the cover to vibrate at ultrasonic frequencies (typically 20 kHz or 40 kHz) while simultaneously applying pressure to the cover. The high frequency vibrations generate enough friction to cause the raised ridge 40 and the portion of the cover in contact with the raised ridge to melt. The pressure applied causes the ridge to flatten and fuse to the cover thereby "welding" the parts together. As illustrated in Figures 3 and 4, the support columns may melt through the membrane and fuse directly to the cover during the ultrasonic welding process. This detailed description is set forth only for purposes of illustrating examples of the present invention and should not be considered to limit the scope thereof in any way. Clearly, numerous additions, substitutions, and other modifications can be made to the invention without departing from the scope of the invention which is defined in the appended claims.

Claims: An ink pen (10) for an ink-jet printer comprising: a reservoir (12) for holding a supply of ink 14; a vent (18) in the reservoir (12) for selectively admitting ambient air into the reservoir (12) to maintain a backpressure within the reservoir (12) within an operating range for the ink pen (10) which allows the ink pen to eject said ink while preventing free flow of said ink from the ink pen, the vent (18) comprising a bubble generator that traps a quantity of said ink within the vent by capillary forces, said trapped quantity of ink sealing the vent (18) when said backpressure is within said operating range and allowing said ambient air to bubble through said trapped quantity of ink and into the reservoir (12) when said backpressure exceeds said operating range to thereby lower said backpressure; an inlet labyrinth (30) coupled to said vent; and a hydrophobic membrane (19) coupled to said inlet labyrinth (30), said hydrophobic membrane (19) allowing the passage of air to said inlet labyrinth and blocking the passage of ink from said inlet labyrinth. An ink pen 10 in accordance with claim 1 in which the bubble generator comprises a capillary member (24) positioned within said vent to trap said trapped quantity of ink within an orifice (10) to seal the vent when the backpressure within the reservoir (12) is within said operating range. An ink pen 10 in accordance with claim 2 in which the orifice (20) is defined by a tubular boss (22) having the capillary member (24) disposed therein. An ink pen in accordance with claim 3, wherein the said first end is in fluid communication with said boss, said inlet labyrinth (30) providing a containment volume for ink that flows through the bubble generator when said backpressure in said reservoir falls below said operating range; and said hydrophobic membrane allowing the passage of air through said second end and into said inlet labyrinth (30) and blocking passage of said ink through said second end to prevent ink from escaping from said inlet labyrinth through said second end. An ink pen in accordance with claims 3 or 4 in which the capillary member (24) is a sphere concentrically disposed within the boss (22) to form the orifice (20). An ink pen in accordance with any one of claims 1 to 5 in which the hydrophobic membrane (19) allows the passage of air at a rate of about 5.5 cubic centimeters per minute per square millimeter with a pressure drop of less than about 1.3 centimeters water column. An ink pen in accordance with any one of claims 1 to 6 in which the hydrophobic membrane (19) prohibits the flow of ink up to a pressure of about (51) centimeters water column. An ink pen in accordance with any one of claims 1 to 7 in which ink is removed from the surface of the hydrophobic membrane when subject to a pressure of less than about 20.4 centimeters water column. An ink pen in accordance with any one of the preceding claims, further comprising: an expandable bladder within said reservoir (12); a spring biasing said expandable bladder to create a backpressure within the reservoir (12).

Inventors: BALDWIN MARC A, DUYCK ELLA M, PLOTKIN LAWRENCE R, BALDWIN, MARC A., DUYCK, ELLA M., PLOTKIN, LAWRENCE R.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1994
Main classification: B41J2
IPC codes: B41J2